A process for producing unsaturated aldehydes and/or unsaturated acids from olefins is a typical example of catalytic vapor phase oxidation.
To perform the partial oxidation of olefins, a multimetal oxide containing molybdenum and bismuth or vanadium or a mixture thereof is used as a catalyst. Typically, the partial oxidation of olefins may be exemplified by a process for producing (meth)acrolein or (meth)acrylic acid by oxidizing propylene or isobutylene, a process for producing phthalic anhydride by oxidizing naphthalene or ortho-xylene or a process for producing maleic anhydride by partially oxidizing benzene, butylene or butadiene.
Generally, propylene or isobutylene is subjected to two-step catalytic vapor phase partial oxidation to form (meth)acrylic acid as a final product. More particularly, in the first step, propylene or isobutylene is oxidized by oxygen, diluted inert gas, water vapor and an optional amount of catalyst to form (meth)acrolein as a main product. In the second step, (meth)acrolein obtained from the preceding step is oxidized by oxygen, diluted inert gas, water vapor and an optional amount of catalyst to form (meth)acrylic acid. The catalyst used in the first step is an oxidation catalyst based on Mo—Bi, which oxidizes propylene or isobutylene to form (meth)acrolein as a main product. Additionally, a part of (meth)acrolein is further oxidized on the same catalyst to form acrylic acid partially. The catalyst used in the second step is an oxidation catalyst based on Mo—V, which oxidizes (meth)acrolein-containing mixed gas produced in the first step, particularly (meth)acrolein, to form (meth)acrylic acid as a main product.
Reactors for carrying out the above process are realized in such a manner that each of the above two steps are implemented in one system or in two different systems (see U.S. Pat. No. 4,256,783).
In general, catalytic vapor phase oxidation is implemented as follows. At least one catalyst in the form of granules is packed into reaction tubes, feed gas is supplied to a reactor through the reaction tubes and the feed gas is in contact with the catalyst in the reaction tubes to perform vapor phase oxidation. Reaction heat generated during the reaction is removed by heat transfer with a heat transfer medium, wherein the temperature of the heat transfer medium is maintained at a predetermined temperature. Particularly, the heat transfer medium for heat exchange is provided on the outer surface of the catalytic tubes to perform heat transfer. A reaction product mixture containing a desired product is collected via a duct and then sent to a purification step. Generally, catalytic vapor phase oxidation is a highly exothermic reaction. Therefore, it is very important to control the reaction temperature in a specific range and to downsize hot spots in the reaction zone.
For example, vapor phase partial oxidation of propylene or isobutylene using a metal oxide catalyst based on molybdenum-bismuth-iron is an exothermic reaction. Therefore, it has a problem in that a hot spot (a point whose temperature is abnormally high) is generated in the reactor. Such hot spots show a relatively high temperature compared to other parts of the reactor. Accordingly, in hot spots, complete oxidation proceeds rather than partial oxidation, thereby increasing by-products such as COx and decreasing the yield of (meth)acrylic acid and (meth)acrolein. Additionally, excessive heat generated in a hot spot causes migration of molybdenum that is a main element of the catalyst, resulting in deposition of molybdenum in a catalytic layer having a relatively low temperature and pressure drop in the catalytic layer, degradation of catalytic activity and in shortening of the lifetime of the catalyst. Therefore, yield of (meth)acrolein and (meth)acrylic acid decreases.
Generally, various methods are known in order to control the excessive heat at a hot spot in a catalytic reaction accompanied with heat generation. Such methods include a method for reducing the amount of feed gas to decrease the space velocity and a method of using a reaction tube having a relatively small inner diameter. However, when the space velocity decreases, it is not possible to obtain high productivity in an industrial scale. When the inner diameter of a reaction tube decreases, it is difficult to manufacture the reactor. Moreover, in the latter case, there are disadvantages of economically unfavorable high cost needed for manufacturing the reactor, and increased time and labor needed for packing a catalyst. For these reasons, there has been a continuous need for and research into a method for producing unsaturated aldehydes and/or unsaturated fatty acids with high yield and high productivity by using a catalyst stably for a long time, while avoiding the above problems according to the known methods.
According to the prior art, disclosed is a reactor for producing unsaturated aldehydes and/or unsaturated fatty acids with high yield over a long time while extending the lifetime of a catalyst, by controlling the excessive reaction heat at the hot spot and optimizing catalytic activity and selectivity. Japanese Laid-Open Patent Nos. Sho53-30688B1 and Hei7-10802A1 disclose a fixed-bed reactor including a reaction zone for the first step of producing acrolein as a main product, the reaction zone comprising a catalytic bed that is formed of a catalyst mixed and diluted with an inactive material and is packed in such a manner that the ratio of the inactive material gradually decreases from the inlet of the reactor toward the outlet of the reactor, i.e., in the direction of reaction gas flow.
U.S. Pat. No. 5,198,581 discloses a fixed-bed multi-tube type reactor for producing unsaturated aldehydes and unsaturated fatty acids by means of catalytic vapor phase oxidation of at least one compound selected from the group consisting of propylene, isobutylene, t-butyl alcohol and methyl-t-butyl ether with molecular oxygen or molecular oxygen-containing gas. The above reactor includes a plurality of reaction zones each packed with a different composite oxide-based catalyst having a different occupation volume along the axial direction of each reaction tube, wherein the volume is controlled so that it decreases from the gas inlet to the outlet. Korean Laid-Open Patent No. 2001-80871 discloses a method for producing acrolein (ACR) and acrylic acid (AA) by means of vapor phase oxidation of propylene with molecular oxygen or molecular oxygen-containing gas in a fixed-bed cylindrical reactor. According to the above method, a plurality of catalysts having different activities are obtained by controlling (a) the volume occupied by a catalyst, (b) sintering temperature, and/or (c) kind and/or amount of alkali metal elements. Additionally, the catalytic bed in each reaction tube is divided into two or more reaction zones along the axial direction, the reaction zones being packed with the catalysts in such a manner that the catalytic activity increases from the reaction gas inlet to the outlet.
As described in the prior art, the method for packing a catalyst after it is mixed and diluted with an inactive material, the method for packing a plurality of composite oxide-based catalysts having different occupation volumes in such a manner that the volume gradually decreases, etc., have problems in that they are inefficient for commercial use because the packing ratio of a catalyst varies depending on the size, shape, specific gravity and density of the catalyst and inactive material, even though the catalyst is mixed and diluted with the inactive material at a correct ratio and then the mixture is packed into a reaction tube. Additionally, the method for packing a catalyst by controlling the catalytic activity through the control of the occupation volume, sintering temperature and/or kind and/or amount of alkali metal elements in the catalyst having a specific composition can reduce the temperature of a hot spot generated during the catalytic reaction, thereby minimizing degradation of catalyst and side reactions. However, the method is problematic in that the hot spot still maintains high temperature.
Therefore, there is a continuous need for a method for minimizing degradation of catalyst and side reactions caused by extreme heat generation at a hot spot generated during the catalytic reaction.